The present invention relates to an improved process for preparing acetylene and synthesis gas by partial oxidation of hydrocarbons in a reactor, in which a stream comprising the hydrocarbon and a stream comprising the oxygen are fed to the reactor, and to an apparatus for performing the process according to the invention.
High-temperature reactions for partial oxidation of hydrocarbons are typically performed in a reactor system composed of mixing unit, burner and quench unit.
One example of such a partial oxidation in the high-temperature range is the preparation of acetylene and synthesis gas by partial oxidation of hydrocarbons. This is described, for example, in DE 875198, DE 1051845, DE 1057094 and DE 4422815.
These documents explain the mixer/burner block/firing space/quench combinations typically used for the BASF-Sachsse-Bartholomé acetylene process—referred to hereinafter, when reference is being made to the combination, simply as “reactor”.
In this process, the starting materials, for example natural gas and oxygen, are heated separately, typically up to 600° C. In a mixing zone, the reactants are mixed intensively and, after flowing through a burner block, reacted exothermically. In these cases, the burner block consists of a particular number of parallel channels in which the flow velocity of the ignitable oxygen/natural gas mixture is higher than the flame velocity (reaction rate, conversion rate), in order to prevent the flame from penetrating into the mixing space. The metallic burner block is cooled in order to withstand the thermal stresses. According to the residence time in the mixing space, the risk arises of premature ignition and reignition owing to the limited thermal stability of the mixtures. The term “ignition delay time” or “induction time” is used here as the period within which an ignitable mixture does not undergo any significant intrinsic thermal alteration. The induction time depends on the type of hydrocarbons used, the mixing state, pressure and temperature. It determines the maximum residence time of the reactants in the mixing space. Reactants such as hydrogen, liquefied gas or light petroleum, the use of which is particularly desirable owing to enhanced yield and/or capacity in the synthesis process, are notable for a comparatively high reactivity and hence short induction time.
The acetylene burners being used on the present production scale are notable for their cylindrical geometry in the firing space. The burner block preferably has hexagonally arranged passage bores. In one embodiment, for example, 127 bores of internal diameter 27 mm are arranged hexagonally on a circular base cross section with diameter approx. 500 mm. In general, the channel diameters used are of diameter about 19 to 27 mm. The downstream firing space in which the flame of the acetylene-forming partial oxidation reaction is stabilized is likewise of cylindrical cross section, is water-cooled and corresponds in terms of appearance to that of a short tube (for example of diameter 180 to 533 mm and of length 380 to 450 mm). At the height of the surface of the burner block on the firing space side, what is called auxiliary oxygen is supplied to the reaction space. This ensures flame stabilization and hence a defined distance of the flame root and hence of the commencement of reaction from the stoppage of reaction by the quench unit. The entire burner composed of burner block and firing space is hung from the top of a quench vessel of relatively large cross section by means of a flange. At the height of the exit plane from the firing space, outside the circumference thereof, quench nozzles are installed in one or more quench distributor rings, which atomize the quench medium, for example water or oil, with or without the aid of an atomization medium, and inject the reaction gases leaving the firing space approximately at right angles to the main flow direction. This direct quench has the task of cooling the reacting flow extremely rapidly to approx. 100° C. (water quench) and 200° C. (oil quench), such that further reactions, especially the degradation of acetylene formed, are frozen. The range and distribution of the quench jets is ideally such that a very substantially homogeneous thermal distribution is achieved within minimum time.
The acetylene burners used on the current production scale are notable for a cylindrical geometry of the firing space. The feedstocks are premixed by means of a diffuser and supplied, with avoidance of backmixing, to the burner block via passage bores in a hexagonal arrangement. In the known processes, the feedstocks are premixed in the mixing diffuser in a relatively large volume and with high preheating temperatures.
The industrial processes described form not only acetylene but essentially hydrogen, carbon monoxide and soot. The soot particles formed in the flame front can adhere as nuclei to the surface on the firing space side of the burner block, which then results in the growth, deposition and baking-on of coke layers, which adversely affects the effectiveness of the process.
In the existing production processes with oil and water quenching, these deposits are periodically removed by mechanical cleaning in the region of the surface on the firing space side of the burner block by means of a stoker unit. For this purpose, complex control of the stoker unit is necessary (Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A1, pages 97-144) and, in addition, the particular use time of the mechanism is limited by the thermal stress in the combustion space.
There has been no lack of attempts to avoid the disadvantage of the baking of coke layers onto the surface on the firing space side of the burner block. For instance, the teaching of DE 2307300 discloses the injection of a gaseous substance into the reactor in a region between maximum temperature and quenching site (claim 1). This is intended to lead to reactions between the gases added and free radicals, which is intended to reduce coke formation (description, page 8, second paragraph).
DE 3904330 A1 describes a process for preparing acetylene black by thermal decomposition of acetylene. It is mentioned in this process, which differs significantly from the process for preparing acetylene (e.g. no partial oxidation), that an inert gas stream is optionally introduced.
DE 1148229 describes a process for operating pyrolysis chambers for treatment of hydrocarbons, wherein purging with steam is provided and cooling of the wall is supposed to lead to a water curtain (claim 1). No further information is given about the way in which the purging is executed. The process presented is not a partial oxidation (POx), the purge medium introduced is liquid water, and additional admixing of an oxidizing agent (e.g. oxygen) with the purge medium is not provided. Furthermore, a purge medium is injected only at a maximum of one site in the axial profile of the pyrolysis chamber.
DE 2007997 describes how an oil film on the interior wall of the reaction chamber is supposed to prevent coking (page 2, first paragraph). However, an oil film in a firing space tends to coking per se. Therefore, a hydrocarbon-containing (mineral) oil can be ruled out as a purge medium given the present challenge.
The processes disclosed in the documents cited for prevention or reduction of unwanted coke formation, however, are unsatisfactory with respect to effective use in the process for preparing acetylene. For instance, some of the documents, as explained, relate to other reactions where the conditions are quite different and there is no applicability. For instance, the partial oxidation in the process according to the invention is very demanding in terms of characteristics: the residence times play a particularly major role, the stoppage of the reaction must be very exact, and the addition of extraneous substances, including, for example, a purge gas or oxidizer, can move the reaction very rapidly with respect to the site and also the rate thereof, thus leading to a yield loss.
In addition, the prior art contains only general statements, often in terms of the problem to be solved, about the technical configuration. Furthermore, the prior art contains explanations with respect to coking on the side surfaces which delimit the firing space, and not on the surface on the firing space side of the burner block, which is typically perpendicular to these side surfaces.
In addition, no geometry specifications in terms of flow mechanics and reaction technology are presented, on the basis of which the purge medium could be minimized effectively and hence operating costs could be minimized and product yield optimized.
It is thus an object of the present invention to find an improved process for partial oxidation of hydrocarbons, which suppresses baked-on and deposited material on the surface on the firing space side of the burner block in a simple manner in terms of process technology, in order that there is no need for mechanical cleaning of this surface on the firing space side of the burner block, and hence for periodic removal by means of mechanical stoker units which are subjected to high thermal stress and are difficult to control.